A conventional refrigerating cycle is described by referring to FIG. 11. In FIG. 11, reference numeral 101 shows a compressor. The refrigerant compressed in the compressor 101 is condensed in a condenser 102. The refrigerant expanded in a throttling unit 103 is evaporated in an evaporator 104, and cooling is effected by evaporation of latent heat.
When operating such a refrigerating cycle, foreign matter mainly composed of iron powder and copper powder mixed at the time of assembly is likely to deposit in the throttling unit 103 where the flow velocity of the refrigerant is slow and the passage area is narrow. Moreover, worn powder from the sliding parts of the compressor and carbides due to deterioration of refrigerating machine oil also deposit in the throttling unit 103. As a result, the sectional area of the throttling unit 103 becomes gradually narrower, the throttling rate becomes larger, and the compression ratio of the high pressure side and low pressure side becomes higher. Accordingly, the temperature of the refrigerant discharged from the compressor is raised, the abrasion of the sliding parts is further promoted, and clogging of the throttling unit 103 with worn powder is increased, thus falling in a spiral. Therefore, the reliability of the refrigerating cycle is spoiled extremely.
As the refrigerant for such refrigerating cycle, hitherto, dichlorofluoromethane (CFC12) or hydrodifluoromethane (HCFC22) has been mainly utilized. As the refrigerating machine oil to be packed in the compressor, naphthene or paraffin mineral oil having compatibility with CFC12 or HCFC22 has been used.
Since these refrigerants and refrigerating machine oils directly circulate within the compressor, the compressor mechanism is required to have wear resistance.
It has been recently disclosed that these refrigerants, when released in the atmosphere, destroy the ozone layer and have serious effects on the human health and ecological system, and therefore the use of CFC12 or HCFC22 is being limited in gradual steps, and there is an international agreement to abolish them completely in the future.
In such circumstance, substitute refrigerants have been developed, such as 1,1,1,2-tetrafluoroethane (HFC134a), pentafluoroethane (HFC125), hydrodifluoromethane (HFC32), and their mixed refrigerants.
These refrigerants HFC134a, HFC125, HFC32 are low in the coefficient of ozone destruction, but are hardly compatible with mineral oils which are refrigerating machine oils employed when using CFC12 or HCFC22. Hence, when using HFC134a, HFC125, HFC32 or their mixed refrigerants as the refrigerant of the refrigerant compressor, it has been attempted to use ester, ether or fluorine oil, compatible with these refrigerants as the refrigerating machine oil.
As the refrigerating machine oil compatible with HFC134a, HFC125, HFC32 replacing the refrigerants CFC12 and HCFC22, polyalkylene glycol oil and polyester oil are known. In the case of the refrigerant compressor using such polyalkylene glycol oil and polyester oil, however, gray cast iron, special cast iron, and stainless steel used as the sliding materials in the compressor are lowered in wear resistance, and the refrigerant compressor cannot be operated stably for a long period.
This is because the chlorine atom, one of the elements composing the conventional refrigerant such as CFC12 and HCFC22, reacts with the iron atom in the metal material and forms a wear resistant iron chloride film. By contrast, when using HFC134a, HFC125, or HFC32 as refrigerant, since the chorine atom is not present in these refrigerants, lubricating film such as iron chloride film is not formed, which is one of the causes of lowering of lubricating action.
Moreover, in the conventional refrigerating machine oil derived from mineral oil, cyclic compounds were contained, and the oil film forming capability was relatively high, but the refrigerating machine oil compatible with HFC134a, HFC125, or HFC32 is mainly composed of chain compounds, and an appropriate oil film thickness cannot be maintained in severe sliding conditions, which also causes to lower the wear resistance.
Thus, in the refrigerant compressor using substitute refrigerant such as HFC134a, HFC125 or HFC32 instead of CFC12 or HCFC22, and employing refrigerating machine oil compatible with these refrigerants, the sliding condition is severe not only at high load but also at ordinary load, and abrasion of sliding members is increased. It was hence a more difficult problem than in the prior art to prevent clogging of the throttling unit in the refrigerating cycle.
Among refrigerating machine oils compatible with HFC refrigerant, polyester derivative refrigerating machine oil undergoes decomposition of polyester due to hydrolysis or pyrolysis, and is bound with worn powder to produce iron soap. The iron soap is high in viscosity, deposits in the throttling unit in the refrigerating cycle, raises the discharge refrigerant temperature in the compressor, and further promotes wear, and the reliability of the refrigerating cycle is lowered by this spiral.
Still more, the refrigerating machine oil compatible with the HFC refrigerant is not compatible with the conventional mineral oil and is not used, but the conventional mineral oil is used as machining oil when fabricating the compressor and heat exchanger. This mineral oil remaining in the refrigerating cycle is likely to deposit in the throttling unit which is slow in flow velocity and drastic in temperature changes. As a result, it leads to decline of reliability due to clogging of the throttling unit same as mentioned above.